The present invention relates to a permanent magnet synchronous machine, and a pressing or extrusion machine using the permanent magnet synchronous machine.
When a permanent magnet synchronous electric machine is rotated at high speed, a voltage drop by inductance increases in proportion to the rotational speed. Therefore, an upper limit of the rotational speed is set in order to prevent a shortage of input voltage. As disclosed in FIG. 4 of JP-A-2002-84690, there is a technique that aims to expand a high-revolving speed range by providing slits in a pole shoe on an outer side and in a pole shoe on an inner side of a permanent magnet embedded in a rotor so as to reduce inductance.
However, with a configuration described in the conventional technique, a radial length of the permanent magnet is greater than a radial length of a rotor core, resulting in an extremely high magnetic resistance. Configurations using such a thick-walled permanent magnet can be found as measures for improving tolerance to demagnetization of a ferrite magnet. In such a configuration, since a major portion of stator flux flows through a pole shoe of a rotor, although slits provided on the pole shoe contribute towards reducing inductance, slits provided on a core part are hardly effective. It is therefore conceivable that, for this reason, JPA-2002-84690 does not clearly specify a quantitative effect. On the other hand, providing slits on the core part reduces flux paths constituted by magnetic material and makes magnetic saturation more likely. Accordingly, there is a problem in that flux generated by the permanent magnet is reduced and, in turn, torque is reduced. While JP-A-2002-84690 avoids any mention of this problem, this is conceivably due to a fact that since a remanent flux density of a permanent magnet is around 0.5 T when using a ferrite magnet, significant magnetic saturation does not occur even when a fair number of slits is provided on the core part.
In contrast, with a rare-earth magnet such as a neodymium magnet, remanent flux density reaches 1 T or higher and magnetic saturation becomes more likely when slits are provided on a rotor. In particular, with a surface permanent magnet synchronous machine, due to a structure thereof, saliency ratio approaches 1 and, in many cases, reluctance torque cannot be sufficiently utilized. Therefore, it is important that torque is generated by a permanent magnet as effectively as possible. For this reason, a method of providing slits on a rotor conceivably causes a decrease in permanent magnet flux and a reduction of torque and is therefore not generally adopted.